With increasing data service requirements, a high-speed fiber optic transmission network with a large capacity gradually becomes a main direction of information transmission. Continuous innovation of new fiber optic communications technologies also contributes to a fiber optic transmission distance multiplied year by year. An optical parameter becomes an important indicator for measuring a fiber optic communications system. To better manage and monitor an optical network, it is necessary to monitor an important transmission parameter of the network, and Optical Performance Monitoring (OPM) gains more attention as fiber optic communications develops. In many parameters, Chromatic Dispersion (CD) is an important parameter that can reflect an operating status of the optical network.
In the fiber optic communications system, optical chromatic dispersion is an important indicator for measuring quality of a fiber link, and is greatly significant to system estimation and measurement. Optical chromatic dispersion indicates a difference between transmission rates of lightwave frequency components. As shown in FIG. 1, an optical signal is carried by different frequency components in an optical fiber, and these different frequency components have different propagation speeds when passing a same medium. This phenomenon is called chromatic dispersion. In terms of time, when an optical pulse propagates through an optical fiber, a waveform of the optical pulse is broadened in time, and therefore causes signal distortion. Consequently, a receive error is caused, and a transmission capacity of the optical fiber is limited. Chromatic dispersion and a fiber length are in a linear relationship, that is, a longer optical fiber indicates stronger chromatic dispersion. A length of an optical fiber through which an optical signal passes can be determined by monitoring CD on an intermediate transmission node in the fiber optic communications system. Alternatively, with a length of an optical fiber known, a chromatic dispersion parameter of the optical fiber can be determined. These monitoring results can provide an important determining basis for evaluating communication quality of the optical network.
In the prior art, a method for measuring chromatic dispersion may be a pulse delay method, and a specific implementation of the method may be (the implementation of the method is shown in FIG. 2):
A pulse signal generator is used to modulate a laser, and an optical signal output from the laser is divided into two signals by using a spectroscope. One signal enters a monitored optical fiber (this optical pulse signal is broadened due to a chromatic dispersion effect). The other signal directly enters an optical monitor and a receiver without passing through the monitored optical fiber. The two received signals are sent to a dual-trace oscilloscope. Widths of the two optical pulses are obtained by respectively measuring displayed pulse waveforms. It is assumed that waveforms of both an optical pulse input into an optical fiber and that output from the optical fiber are approximately Gaussian, and chromatic dispersion of the optical fiber can be calculated by measuring, by using a time domain method, pulse broadening caused by fiber optic transmission.
The pulse delay method is a method in which a fiber chromatic dispersion coefficient is directly obtained from a defined formula by measuring a time delay difference between narrow optical pulses with different wavelengths after the narrow optical pulses are transmitted through an optical fiber. In this method, a narrow optical pulse with a known shape (a width is generally several hundred ps) is injected into a to-be-monitored optical fiber. The optical pulse is broadened due to chromatic dispersion of the optical fiber after being transmitted along the optical fiber. A waveform of this broadened optical pulse is recorded at an output end of the optical fiber. Pulse broadening caused by the chromatic dispersion can be obtained from a difference between a width of an output pulse and a width of an input pulse, and therefore the chromatic dispersion caused by the optical fiber can be obtained by means of estimation according to the broadening.
It can be learned from the implementation of the foregoing pulse delay method that, a comparison between original pulses is required to estimate chromatic dispersion by using this method, and this is difficult to implement in actual long-distance transmission application.